Atomic beam tube having multiple beams

ABSTRACT

An atomic beam tube produces a plurality of atomic beams, all of which are passed through a pair of R.F. cavities and then detected by a single detector. First and second state-selecting magnets are configured with a plurality of deflecting gaps, each of which produces an inhomogeneous magnetic field. The deflecting gaps are arranged in symmetrical pairs to provide a beam tube having a high tolerance to acceleration forces. The use of multiple beams provides a high signal-to-noise ratio.

Unite States Cutler, executor et al.

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[ July4, 1972 [54] ATOMIC BEAM TUBE HAVING MULTIPLE BEAMS [72] Inventors: Leonard S. Cutler, executor, Los Altos Hills, Calif.; Joseph H. Holloway, deceased,

late of Saratoga, Calif.; Wilson R. Turner,

[21 Appl. No.: 85,092

Related US. Application Data [63] Continuation-in-part of Ser. No. 743,839, July I0,

1968, abandoned.

521 US. Cl ..331/94 330/4 The deflecting gaps are arranged Symmetrical Pairs to P [51] Int CL i 1/06 vide a beam tube having a high tolerance to acceleration [58] Field 330/4 forces. The use of multiple beams provides a high signal-tonoise ratio- [56] References Cited UNITED STATES PATENTS 6 Claims, 6 Drawing Figures 3,113,207 12/1963 Bederson et al. ..331/3 X r fi :4 50 95) '46 OTHER PUBLICATIONS Marton, Advances in Electronics and Electron Physics, Academic Press Inc., New York, N.Y., I956, pp. 4- 9. Holloway et al., Comparison and Evaluation of Cesium Atomic Beam Frequency Standards," Proc. IRE, Oct. 1959, pp. 1730-l736.

Primary Examiner Roy Lake Assistant ExaminerSiegfricd H Grimm Att0rneyStcphcnP. Fox

[5 7] ABSTRACT An atomic beam tube produces a plurality of atomic beams, all of which are passed through a pair of RP. cavities and then detected by a single detector. First and second state-selecting magnets are configured with a plurality of deflecting gaps, each of which produces an inhomogeneous magnetic field.

Patented July 4, 1972 3,575,149

2 Sheets-Sheet l mumnmmmm llll' iqure 3 INVENTORS LEONARD S. CUTLER JOSEPH H. HOLLOWAY WILSON R. TURNER AT TORNEY Patented July 4, 1972 3,675,149

2 Sheets-Sheet 2 INVENTORS LEONARD S. CUTLER JOSEPH H. HOLLOWAY WILSON R. TURNER BY A? ATTORNEY q Fiure 40 ATOMIC BEAM TUBE HAVING MULTIPLE BEAMS This is a continuation-in-part of patent application Ser. No. 743,839 filed July 10, 1968 and now abandoned.

BACKGROUND OF THE INVENTION Atomic and molecular beam tubes find present-day use as the basic reference elements in extremely stable frequency standards in, for example, systems for precisely measuring time and/or frequency. Basically, the beam tube comprises a beam source for generating the atomic beam, a first deflecting or state-selecting magnet, commonly referred to as the A magnet, through which the beam is passed for selecting the atom particles from one of the desired energy states in which the atom particles in the beam exist for transmittal into the radio frequency transition section of the tube. In the radio frequency transition section of the tube the atom particles undergo magnetic hyperfine resonance transitions, i.e., transitions from one energy state to the other. This is accomplished by applying radio frequency energy to the atom particles at the transition frequency of the particular particles in the presence of a uniform magnetic C field of proper orientation relative to the R.F. field, this magnetic field being of low value,,for example one twentieth of a gauss, relative to the A magnet which may be, for example, kilogauss. The atom particles pass out from the radio frequency transition section of the tube into a second deflecting or state-selecting magnet, commonly referred to as the B magnet, which is similar to the A magnet. The atom particles which have undergone R.F. transition will be directed by the B magnet field onto a suitable beam detector as an indication that magnetic resonance has occurred, whereas, in the absence of resonance transitions, the beam will be caused to miss the target or detector.

An atomic beam tube will operatesatisfactorily as a precision frequency standard when it produces a detector output having a high signal-to-noise ratio. Commercial atomic beam tubes presently available employ a single beam path having a length between the oven source and detector of at least inches. For reasons of economy in cost and space, it is desirable to minimize the overall length of the beam tube. However, attempts at shortening the beam path produce poor results because the signal decreases which in turn causes a decrease in the signal-to-noise ratio. This drop in the signal-to-noise ratio cannot be eliminated by increasing the cross-sectional diameter of the beam because the state-selecting magnets do not sufficiently deflect the wider beam.

Atomic beam tubes are often used in navigating systems in aircraft or other mobile applications. In such applications, the beam tube may be subjected to acceleration and Coriolis forces which shift the beam path relative to the deflecting magnets, the radio frequency transition section, and the beam detector. The resulting misalignment causes intensity changes and transients to occur in the beam tube output signal which adversely affect accuracy of the tube in operation.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of this invention to improve the signal-tonoise ratio of an atomic beam frequency standard tube of a given length.

It is another object of the invention to decrease the length of an atomic beam tube without undesirable efiects on the signalto-noise ratio of the output.

It is a still further object of the invention to reduce the adverse effects of accelerafion and Coriolis forces which may act on an atomic beam tube.

These objects are attained by an atomic beam tube structure using a plurality of parallel beams. The parallel beams are formed by a state-selecting magnet having a plurality of deflecting gaps, each of which produces an inhomogeneous magnetic field. The deflecting gaps are arranged in axially symmetrical pairs spaced apart laterally of the main longitudinal axis of the parallel beams. In an illustrated embodiment of the invention, the symmetrical deflecting gaps are arranged in two rows spaced apart from one another so that the plurality of beams formed lie in two planes which are parallel to one another and also parallel to an axis perpendicular to the main axis of the beam tube.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating the principles employed in a typical prior art atomic beam tube.

FIG. 2 is a diagrammatic illustration of one embodiment of the invention.

FIG. 3 is an end view, rotated clockwise, of the collimator taken along line 3-3 of FIG. 2.

FIGS. 4a, b are cross-sectional views, rotated 90 clockwise, taken along line 4-4 of FIG. 2 and illustrating different constructions of the state'selecting magnet means.

FIG. 5 is a diagrammatic perspective view of an atomic beam tube illustrating the paths of the beams and typical acceleration forces which may act on the tube.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a typical prior art beam tube is provided with a chamber or source oven 2 at one end and the detector 12 at the other end. The atoms projected from the source oven 2 successively undergo interaction with the A magnetic field produced in state-selecting magnet 4, the weak homogeneous C magnetic field and the oscillating field in the central portion 20, and finally the B magnetic field produced by state-selecting magnet l I.

The detector 12 and the chamber 2 are offset from the longitudinal axis of the beam tube. Magnet 4 produces an in homogeneous field, and the stream of atoms passing therethrough is split, so that those atoms having predetermined energy levels are deflected through the tube. The atoms whose energies cause them to be discarded collide with the walls of the tube and are absorbed by suitable getters. Those atoms which continue through the tube are subjected to the oscillating electromagnetic fields in cavities 8 and 10, and if the applied wave is at the correct resonant frequency, changes in energy level take place. The inhomogeneous magnetic field produced by the B magnet 11 has approximately the same strength, and the space gradient is oriented in the same direction, as the A magnet 4. This results in discarding all atoms except those which have undergone a transition in state, the latter of which pass to the ionizer-detector 12. The magnitude of the detector current depends on frequency, and after suitable amplification, the detector current is utilized to drive a servo system to control the frequency of the generator 14 from which the cavities 8 and 10 are excited. The control system is operated to search for the maximum value of the detector current and to hold the average frequency of the R.F. excitation of the cavities at the frequency represented by predetermined transitions of the atoms.

FIGS. 2 and 4a illustrate one embodiment of an atomic beam tube incorporating the multiple beams of the present invention. Specifically, FIG. 2 shows a multiple beam tube comprising a source 38, first state-selecting magnet means 40, a radio frequency transition section 42, second stateselecting magnet means 44, and a detector 46. The source 38 includes a collimator 48 having two collimating slits 50 and 52 for directing two streams of atoms to the first state-selecting magnet means 40. FIG. 3 is an end view of the collimator and illustrates the arrangement of the collimating slits. The general operation of this beam tube is similar to that described above with respect to FIG. 1.

The preferred configuration of the first state-selecting magnet means 40 is shown in detail in the sectional view of FIG. 4a. There are provided first and second pole pieces 54 and 56 constructed of soft magnet iron and secured respectively to permanent magnets 58 and 60. The strength of the field produced by magnets 58, 60 should be high, on the order of 10,000 gauss. A U-shaped iron frame member 62 secures the permanent magnets and pole pieces in juxtaposed relation and provides a return path for the magnetic flux. A third soft iron ole piece 64 is supported between the two pole pieces 54, 56 by nonemagnetic members 66 and 68 which are brazed or otherwise joined to the ends of the pole pieces. As shown, the pole pieces 54 and 56 are configured with six vertically spaced convex portions and the pole piece 64 is configured with twelve concave portions respectively located adjacent to the convex portions of pole pieces 54, 56, so as to define twelve deflecting gaps 70 each of which produces an inhomogeneous magnetic field. These twelve deflecting gaps are arranged symmetrically in six pairs. As shown in FIG. 4a, the symmetrical deflecting gaps of each pair are spaced apart horizontally and the six pairs of deflecting gaps form two vertical rows. A top elevation of these two rows is illustrated diagrammatically in FIG. 2. The magnetic polarities induced in the pole piece 64 are indicated by the letters S and N, in the case where permanent magnets 58 and 60 are respectively north and south poles, as shown.

The two streams of atoms effusing from the collimator slits 50, 52 are directed to the selecting magnet 40 shown in FIG. 4a and formed into 12 atomic beams which travel in parallel paths through the radio frequency transition section 42. Thereafter the atoms of the 12 beams pass through the second state-selecting magnet means 44 having the same configuration as selecting magnet 40 and being axially aligned therewith to receive the twelve beams through the 12 deflecting gaps, respectively. The atoms from all 12 beams which have undergone a transition in state are deflected by their corresponding deflecting gap in a direction such that they impinge on the detector 46.

Another configuration for the state-selecting magnets 40 and 44 is shown in FIG. 4b. Pole pieces 72 and 74 have inwardly facing concave portions arranged opposite one another in vertically spaced pairs. Between the pairs of concave portions there are positioned soft iron rods 76 supported by nonmagnetic members 78. Four deflecting gaps 80 are arranged symmetrically in two rows, each row having two deflecting gaps.

The number of deflecting gaps illustrated in FIGS. 4a and 4b is merely exemplary, and other configurations may be used, providing that there are at least two deflecting gaps and that the gaps are arranged in symmetrical pairs. More specifically, the deflecting gaps of each pair should be axially symmetrical with respect to an axis which is longitudinal of the beam tube (i.e. an axis parallel to the axis of the atomic beams) and which is also disposed centrally between the pair of deflecting gaps. In FIGS. 4a and 4b, the axes of symmetry are perpendicular to the plane of the paper and they are exemplified by the dots 81 for the upper pairs of deflecting gaps.'

The axial symmetry of each pair of deflecting gaps causes the atoms passing therethrough to be deflected in opposite directions. For example, with reference to FIG. 4a, the horizontally opposed deflecting gaps are a symmetrical pair and there are six such symmetrical pairs forming two vertical rows. The atoms which pass through magnet 40 are deflected so that the two rows of selected atoms are caused to diverge into parallel beams, whereas the beams which pass through magnet 44 are deflected so that the two rows converge toward detector 46.

It has been found that when the deflecting gaps are arranged in symmetrical pairs and spaced apart laterally with respect to the axis of the beams, as described above, the beam tube has the capability of withstanding substantial acceleration forces without seriously affecting the operation thereof. FIG. illustrates the relationship between the parallel beams and the acceleration forces which may act on the beam tube. The beam tube is represented diagrammatically by the cylinder 82, and the plurality of atomic beams produced are in two planes 84, 86, corresponding respectively to the two rows of beams. The beams travel in the direction of the main longitudinal axis 88 of the tube. The two planes 84, 86 are parallel to one another and also parallel to an axis 90 which is perpendicular to the main axis 88. The acceleration forces which would adversely affect the operation of heretofore known beam tubes are rotational forces about the axis 90, as indicated by the arrow 92. However, with the above-described symmetrical deflecting gap configuration which produces multiple beams, the effect of such acceleration forces is minimized. The reason for this capability is that the efiects of a momentary lateral displacement and attendant change in intensity of one beam of a pair is accompanied by a compensatory change in intensity of the other beam of the pair.

The signal-to-noise ratio at the output of the multiple beam tube is improved by a factor of the number of beams used, in accordance with the following expression:

)k )1 V K where (S/N) is the signal-to-noise ratio of a given atomic beam tube having a single beam, (S/N) is the signal-to-noise ratio of a multiple beam tube having the same general dimensions as the single beam tube, and K is the number of beams in the multiple beam tube.

The embodiments of the invention illustrated and described herein should not be construed in a limiting sense. For example, although the invention has been described with reference to atomic beams, it is to be noted that molecular beams may be used, provided that the molecules have the desired transition characteristics. The scope of the invention further comprehends a multiple beam tube in which the radio frequency section produces electric resonance instead of magnetic resonance. Also, the first and second state-selecting means may incorporate the principles of electrostatic deflection instead of magnetic deflection. When electrostatic deflection is used, the first and second state-selecting means are configured with a plurality of electric dipoles, each of which produces an electric field gradient for deflecting the atoms or molecules. Finally, although the illustrated embodiments use a dual slit collimator and a single detector, it is to be noted that alternatively, there could be used a single collimator slit aligned midway between the two rows of deflecting gaps, and two detectors, one for each row of beams.

We claim:

1. A molecular beam tube apparatus which can withstand substantial external acceleration forces comprising:

a source for projecting molecular particles;

first state-selecting means for forming said projected molecular particles into a plurality of parallel molecular beams;

a radio frequency transition section including means for producing a uniform C field, said transition section being disposed downstream from said first state-selecting means in the paths of said plurality of parallel molecular beams;

second state-selecting means disposed downstream from said radio frequency transition section and aligned with said plurality of parallel molecular beams for deflecting selected molecular particles in each of said beams; and

detector means for receiving molecular particles from said second state-selecting means to indicate when molecular resonance occurs;

said first and second state-selecting means each including a plurality of deflecting gaps corresponding respectively to said plurality of molecular beams, each of said deflecting gaps being configured to produce an inhomogeneous field;

said plurality of deflecting gaps being arranged in pairs,

each pair of deflecting gaps being con-figured symmetrically with respect to an axis disposed centrally therebetween and longitudinal to the beam paths;

whereby any lateral displacement and attendant change in intensity of an atomic beam passing through one deflecting gap of a pair is minimized by a compensatory change in intensity of the atomic beam passing through the other deflecting gap of a pair.

2. The apparatus of claim 1 wherein said first and second state-selecting means have substantially the same configuration and each includes twelve deflecting gaps arranged in six symmetrical pairs.

3. The apparatus of claim 1 wherein said first and second state-selecting means have substantially the same configuration and each has two deflecting gaps arranged in a symmetrical pair.

4. An atomic beam tube apparatus comprising: source means for projecting atomic particles along a path;

a first state-selecting magnet aligned with the path of atomic particles and including pole pieces defining multiple deflecting gaps, said deflecting gaps being arranged in two parallel rows to form a plurality of atomic beams in parallel planes, each deflecting gap in one of said rows being symmetrical to a corresponding deflecting gap in the other of said rows;

a radio frequency transition section disposed downstream from said first state-selecting magnet and in alignment with said plurality of atomic beams, said radio frequency transition section including means for producing a uniform magnetic C field; and

a second state-selecting magnet disposed downstream from said radio frequency transition section and in alignment with said plurality of atomic beams, said second stateselecting magnet having substantially the same configuration as said first state-selecting magnet; and

detector means for receiving atomic particles from said second state-selecting magnet to indicate when magnetic resonance occurs.

5. The apparatus of claim 4 wherein said deflecting gaps defined by said pole pieces are spaced apart laterally of a longitudinal axis of said beam tube and said parallel planes are parallel to said longitudinal axis and to an axis perpendicular to said longitudinal axis, whereby substantial acceleration forces acting on the beam tube about said perpendicular axis do not seriously affect the operation thereof.

6. The apparatus of claim 4 wherein each of said first and second state-selecting magnets includes twelve deflecting gaps in six symmetrical pairs. 

1. A molecuLar beam tube apparatus which can withstand substantial external acceleration forces comprising: a source for projecting molecular particles; first state-selecting means for forming said projected molecular particles into a plurality of parallel molecular beams; a radio frequency transition section including means for producing a uniform C field, said transition section being disposed downstream from said first state-selecting means in the paths of said plurality of parallel molecular beams; second state-selecting means disposed downstream from said radio frequency transition section and aligned with said plurality of parallel molecular beams for deflecting selected molecular particles in each of said beams; and detector means for receiving molecular particles from said second state-selecting means to indicate when molecular resonance occurs; said first and second state-selecting means each including a plurality of deflecting gaps corresponding respectively to said plurality of molecular beams, each of said deflecting gaps being configured to produce an inhomogeneous field; said plurality of deflecting gaps being arranged in pairs, each pair of deflecting gaps being con-figured symmetrically with respect to an axis disposed centrally therebetween and longitudinal to the beam paths; whereby any lateral displacement and attendant change in intensity of an atomic beam passing through one deflecting gap of a pair is minimized by a compensatory change in intensity of the atomic beam passing through the other deflecting gap of a pair.
 2. The apparatus of claim 1 wherein said first and second state-selecting means have substantially the same configuration and each includes twelve deflecting gaps arranged in six symmetrical pairs.
 3. The apparatus of claim 1 wherein said first and second state-selecting means have substantially the same configuration and each has two deflecting gaps arranged in a symmetrical pair.
 4. An atomic beam tube apparatus comprising: source means for projecting atomic particles along a path; a first state-selecting magnet aligned with the path of atomic particles and including pole pieces defining multiple deflecting gaps, said deflecting gaps being arranged in two parallel rows to form a plurality of atomic beams in parallel planes, each deflecting gap in one of said rows being symmetrical to a corresponding deflecting gap in the other of said rows; a radio frequency transition section disposed downstream from said first state-selecting magnet and in alignment with said plurality of atomic beams, said radio frequency transition section including means for producing a uniform magnetic C field; and a second state-selecting magnet disposed downstream from said radio frequency transition section and in alignment with said plurality of atomic beams, said second state-selecting magnet having substantially the same configuration as said first state-selecting magnet; and detector means for receiving atomic particles from said second state-selecting magnet to indicate when magnetic resonance occurs.
 5. The apparatus of claim 4 wherein said deflecting gaps defined by said pole pieces are spaced apart laterally of a longitudinal axis of said beam tube and said parallel planes are parallel to said longitudinal axis and to an axis perpendicular to said longitudinal axis, whereby substantial acceleration forces acting on the beam tube about said perpendicular axis do not seriously affect the operation thereof.
 6. The apparatus of claim 4 wherein each of said first and second state-selecting magnets includes twelve deflecting gaps in six symmetrical pairs. 